1. Field of the Invention
The present invention relates to a method of detecting labels present in a detection zone and exposed to a magnetic bias field in said zone, each label comprising a set of amorphous magneto-elastical strips, preferably at least two strips, said method comprising the steps of bringing the strips into oscillation by means of an excitation field and detecting the resonance frequencies of the strips.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98.
WO-A-93/14478 relates to a method and a device for remote sensing of objects. The objects are marked with at least one label comprising at least one electrical resonant circuit having an induction means and a capacitor means. The resonant circuit is excited to resonance at a resonant frequency, which is detected by the electromagnetic energy transmitted from the resonant circuit. An element of magnetic material having a varying permeability is inductively coupled to said induction element. The resonant frequency of the resonant circuit is affected by the permeability of the element of magnetic material, and this element of magnetic material is exposed to an external and spatially heterogeneous magnetic bias field through which the permeability of the element of magnetic material is controlled.
In the Swedish patent application No 9401449-5, which has not been published yet, there is shown a method of detecting labels with amorphous magneto-elastical strips, said strips being arranged in each label with a certain angular displacement between each other. According to the description of said patent application the following steps are taken in order to detect a label, having an unknown combination of angles. The elements in the label are forced into self-oscillation, their resonance frequencies thereby being detectable. The resonance frequency of an element or a strip is a function of the magnetic field or the magnetizing field strength exposing the element, and hence the magnetic field exposing each element may be determined. It should be noted that the magnetic field is a projection of the actual magnetic field along the element length extension; said magnetic field thereby constituting a component of the actual magnetic field.
There is no way of knowing beforehand which of the strips that belong to each respective label, and hence every possible combination of strips that could form a label must be tested, and for any such combination all angles must be searched so as to find out whether the combination in question constitutes a real label, i.e. a correct combination of strips forming a certain code.
Therefore, all possible combinations of magnetic field components are computed in a calculating unit. Possible combinations are determined by the predetermined sets of angles. Every component pair is used to put together a possible, actual magnetic field vector. Every possible combination provides several magnetic field vectors, and taken together the number of possible magnetic field vectors becomes large. In other words, all possible differential angles between all code strips in a label are computed and matched against each other with respect to a projected magnetizing field strength H.
All elements in a label are exposed to the same H-vector, and therefore each correct code combination among all the determined ones should give the same H-vector value. Every combination of codes or angles giving the same H-vector therefore may constitute a real label combination.
The above method is then repeated with different magnetic field conditions so as to filter out any incorrect (i.e. not real) combinations. Every new condition (strength, gradient, or direction) means several conditions that must be fulfilled for the combination in question to be correct. After a number of repetitions only the correct (real) codes remain, all labels present in the detection zone, even multiple ones with identical code sets, thereby being detected.
In the method described in said patent application every single decoding requires a substantial amount of calculation work, which must be repeated numerous times so as to obtain an accurate detection, and this is particularly true if the number of labels in the detection zone is large.